Boron nitride is a compound which comes in a "soft" form and in two "hard" forms. In the "soft" form it is a material which crystallizes in the hexagonal system and cleaves readily in a manner similar to graphite and molybdenum disulfide. Like these materials it is a good dry lubricant.
If boron nitride is subjected to ultrahigh pressures and elevated temperatures, it is converted to a cubic crystal similar to the crystal of zincblende to produce one of its "hard" forms. The preparation of this form of boron nitride is disclosed and claimed in Wentorf U.S. Pat. No. 2,947,617 which is assigned to the same assignee as the present invention.
If "soft" boron nitride is subjected to pressures of at least about 113 kilobars preferably at a temperature somewhat higher than room temperature, it is converted to a densely packed form of "hard" boron nitride possessing the same hexagonal crystal structure as the mineral wurtzite. This form of "hard" boron nitride is disclosed and claimed in Bundy et al. U.S. Pat. No. 3,212,851 which also is assigned to the same assignee as the present invention.
The zincblende form of boron nitride was discovered some years before the wurtzite form. Both the zincblende and wurtzite forms are useful in the practice of this invention and it is intended that the wurtzite form be included in the term "cubic boron nitride".
The discovery of cubic boron nitride occurred several years after the discovery of a reproducible synthesis for diamond. Initially it was thought that cubic boron nitride was as hard as diamond but additional investigations revealed that cubic boron nitride is a close second to diamond in hardness. Both are considerably harder than other abrasive materials.
Synthetic diamond became commercially available as an abrasive in 1957. It was early established that the abrasive qualities of synthetic diamond were superior to those of natural diamond. The disparity in favor of synthetic has steadily increased as processes for tailoring the product to specific applications have been developed.
From the inception of its commercialization, synthetic diamond has been a growing industry. Its use in a resin-bonded wheel for grinding tungsten carbides has brought about great economies in the finishing of carbide tools. Its use in a metal bond has resulted in saws which bring about great improvement in the cutting of natural stones and ceramics.
When cubic boron nitride was first discovered, it was thought that it would have widespread usage due to its hardness and other properties. For instance, cubic boron nitride can withstand temperatures of 2,500.degree. F. whereas diamond begins to burn at 1,600.degree. F. Nevertheless, cubic boron nitride proved to be inferior to diamond as an abrasive for tungsten carbides, ceramics, and natural stones. It offered advantages over diamond in the cutting of hardened steels and steel alloys but neither diamond nor cubic boron nitride was competitive with such abrasives as aluminum oxide for hardened steels and steel alloys. Thus, although cubic boron nitride is 21/2 times as hard as aluminum oxide, its cost per gram would inherently be about the same as the cost per kilogram of aluminum oxide. Even though other cost factors such as labor, wheel dressing, etc., favored cubic boron nitride, the disparity in material cost was too great for cubic boron nitride to compete in the aluminum oxide market.
In 1967, metal-coated diamond was introduced to the market for use in resinoid grinding wheels. Typically, the use of metal-coated diamond extended the effectiveness of grinding wheels by a factor of two. Such wheels are described in Lindstrom and Lundblad South African Pat. No. 66/5310 advertised on Mar. 8, 1967 and assigned to Allmanna Svenska Elektriska Aktiebolaget. Another document describing metal-coated diamond resinoid wheels is Sacco South African Pat. No. 67/2576 of the Norton Company.
The Sacco patent pointed out that it had been known to coat aluminum oxide abrasive grains with nickel to achieve improved bonding of abrasive particles in a resinoid wheel. In addition, the patent postulated that the greater heat conductivity of heat capacity of diamond coated with metal prevented rapid deterioration of the resin matrix in the vicinity of the abrasive grit thereby allowing for longer retention of the abrasive grit in the resin matrix.
The Sacco patent gave examples of five resinoid wheels using diamond abrasives coated with various metals. The improvement in grinding ratio versus wheels made with uncoated diamond varied from 37 percent to 280 percent. Thus, the most-improved wheels removed 3.8 times as much material from workpieces as did wheels made with uncoated diamond from similar workpieces. These levels of improvement are typical of what has been achieved by metal-coated diamond.